Development of an atom-cluster generator for an ultra-high voltage electron microscope

点击翻译按钮获取中文摘要

When the size of a solid decreases to as small as several nm, its physical and chemical properties change remarkably as compared to those of a bulk solid. Hereafter such ultrafine particles are designated as atom clusters or simply clusters. However, studies on the diffusional properties in atom clusters are lacking. Based on this premise, in the present work, an atom-cluster generator for UHVEM was developed, and with the use of it, alloying behaviour in atom clusters was examined. The generator consisted of two spiral-shaped filaments made of tungsten. The generator was set in the specimen chamber of a 3MV HVEN. The distance between the filaments and a supporting film (substrate) for atom clusters was approximately 100 mm. An amorphous cabon film was used as the supporting film. With the use of the generator gold clusters of -3nm in size were first deposited on the supporting carbon film. Copper clusters were then formed on the same film. Change in the morphology and lattice constant of the clusters associated with the copper cluster deposition were studied in situ by transmission electron microscopy. It is revealed that upon the deposition of copper a rapid dissolution of copper into gold clusters has taken place. The observed copper diffusivity in gold clusters is approximately 9 orders of magnitude faster than that in bulk gold. It is also revealed that the center of gravity of the gold clusters has shown no visible translational motion during the alloying. These observations clearly indicate that deposited copper has either directly or after migration on the substrate, impinged upon and joined with gold clusters and has dissolved quickly into the interior of the gold clusters to form homogeneously mixed Au-Cu alloy clusters. This dissolution has taken place without visible translational motionof individual clusters on the supporting film.